Controlling ectoparasite infestations on naturally occurring small mammal hosts may be an efficient means of protecting humans from several important health threats. Many arthropod species, including a variety of ticks, serve to transmit disease from their natural hosts to man.
Present methods for managing infestations of disease-transmitting ticks to protect the public health are few and suffer from various drawbacks. For example, acaricidal treatment of vegetation is impractical because of the quantity of environmentally objectionable chemicals required and the extent of the region to be treated. Despite this drawback, commercial pest control applicators have attempted such treatment locally to control ticks and fleas, but without any reported success. In special situations, burning of vegetation or other environmental modification may hold some promise, but further evaluation is required before this can be considered a practical alternative. Individual residents of affected regions may gain some personal protection by using tick or other insect repellents (Schreck et al., 1982b) but this measure requires discipline, and the necessary materials are not generally available.
Because of the difficulty in attacking ectoparasitic arthropods directly, present efforts, as for example is the case with a group of related tick species, are focused on reducing the abundance of mammal species, particularly deer, serving as hosts for the reproductive stage ticks. This measure is controversial because of the potential ecological impact of such a program. Efforts to reduce the abundance of rodents, another mammalian host, seem counterproductive. Apparently, reducing the abundance of rodents, even if it were practical, may serve to increase the density of ticks relative to these hosts, which in turn would increase the risk of human infection (Spielman et al., 1981)
Insecticides for control of ectoparasites and other hematophagous arthropods have been applied directly to small native rodents.
There is recorded in the literature a method for attacking sandflies by distributing small pieces of cloth impregnated with insecticide near the burrows of desert rodents; rodents take the pieces of cloth into their burrows, and thus kill the sandflies. However, this approach requires precise knowledge of the location of the burrows, and this is impracticable under actual field conditions. Wild rodent hosts of fleas have been treated using the "bait box" technique (Kartman, 1958, 1960). In this treatment, rodents were attracted to baited containers where they dusted themselves with pesticide-impregnated powder. Modifications of the bait box technique have recently been described, extending treatment to tick-infested rodents (Sonenshine and Haines, Sept. 20, 1985). Thus, such treatment provides for immediate reduction of the arthropod infestation, but continuous self-treatment by rodents is necessary to destroy all vectors which would feed on a host in its lifetime.
Health threats transmitted by ectoparasites of rodents include Lyme disease and babesiosis (both transmitted by Ixodes ticks), Rocky Mountain spotted fever (transmitted by Dermacentor ticks), scrub typhus (also called tsutsugamushi disease) (transmitted by chiggers, especially in Asia but also in Africa), murine typhus and plague (both transmitted by fleas), and rickettsial pox (transmitted by gamasid mites).
In order to illustrate the types of problems posed in dealing with diseases of the foregoing nature, Lyme disease, the most common form of tick-borne disease in the United States, is discussed in detail.
Lyme disease is a complex multisystem disorder involving neurological or cardiac abnormalities as well as intermittent attacks of arthritis. If untreated, the disease can become severe: arthritis can become chronic, with erosion of bone and cartilage. Treatment in the later stages of the disease is not always successful and involves costly hospitalization. Early treatment of Lyme disease with antibiotic therapy generally is successful (Steere, 1983b); however, disease symptoms even in endemic regions often go unrecognized or misdiagnosed. Regardless of the antibiotic therapy given, patients still may experience minor late complications, such as recurrent episodes of pain in joints, bursae or tendons (steere et al., 1983b). More serious tertiary manifestations of Lyme disease, involving delayed neurological effects (similiar to those of tertiary syphillis) have recently been reported. In the decade since its discovery, Lyme disease has presented a growing and significant threat to the public health.
Throughout most of the regions affected by Lyme disease, the vector is a recently described deer tick, Ixodes dammini (Spielman et al., 1979; Wallis, 1978). The causal agent is a spirochete (Burgdorfer et al., 1982), recently designated Borrelia burgdorferi (Johnson, et al., 1984). White-footed mice (Peromyscus leucopus) appear to be the effective reservoir hosts (Levine et al., 1985) since mice remain infective for long periods of time (Donahue, unpublished), and most immature I. dammini feed on these animals (Piesman and Spielman, 1979). Zoonotic transmission of B. burgdorferi requires that larvae acquire the pathogen in an infective blood meal, and derived nymphs transmit the infection back to mice. Human infection usually results from the bit of an infected nymph.
Lyme disease apparently is spreading geographically and increasing in incidence. The number of cases reported to the Center for Disease Control (CDC) has increased over the past two years. Lyme disease is now the most commonly reported tick-borne illness in the United States (Anon, 1985). Nearly 1,500 documented infections were recorded by the CDC during 1984 from 21 states. Of course, undiagnosed, incompletely documented and unreported cases should greatly outnumber these reported cases. Further, additional seemingly inapparent infections occur. For example, 40 percent of residents ages 8-82 in one Massachusetts summer community were sero-positive for Lyme disease, yet only 25 percent of infected individuals reported acute clinical symptoms (unpublished). Interestingly, nearly 30 percent of these residents indicated having intermittent arthritic attacks or other later manifestations of Lyme disease. Given the possibility of tertiary effects from even these inapparent infections, this disease is a serious matter.